The invention relates to a method for desulphurizing an exhaust-gas recirculation flow in the case of an internal combustion engine and to a device for carrying out the method.
Exhaust-gas recirculation is used in internal combustion engines in order to lower NOx emissions, wherein there is the problem that, if fuel containing high levels of sulphur is used, sulphur dioxide and, within the cooled exhaust-gas recirculation arrangement, sulphuric acids and/or sulphurous acids are generated which lead to severe corrosion of the engine components such as charge-air pipe, inlet valves, cylinder liners etc.
In technical terms, exhaust gases are normally desuiphurized through the addition of CaOH or CaO and the subsequent formation of calcium sulphate (DE000003603365C2). However, the use thereof in the EGR line is difficult because CaO, CaOH and the CaSO4 that forms are relatively highly abrasive, which leads to severe wear of the cylinder liners.
Desulphurization is also possible by means of NH3 with the formation of ammonium sulphate (DE 3636554 A1), but this has failed to become established owing to the high costs for the NH3 that is used and the cumbersome process management:2NH3+SO3+H2O→(NH4)2SO4 NH3+SO3+H2O→(NH4)HSO4 
Thus, if this method is used, the exhaust gas temperature must be lowered to values below 300° C., advantageously to values below 100° C., because above said values, ammonium sulphate or ammonium hydrogen sulphate respectively breaks down again or does not form at all.
On the other hand, by means of selective catalytic reduction (referred to as the SCR method for short), it is possible for nitrogen oxides to be catalytically reduced downstream of the internal combustion engine with the aid of NH3. Usually, in vehicles, instead of NH3, use is made of urea which releases NH3 in the hot exhaust gas. This breakdown can be improved through the use of a so-called hydrolysis catalytic converter such as is described for example in DE 4038054.
EP1052009 furthermore discloses a method for carrying out the breakdown of urea in a partial flow.
It is basically possible, in an exhaust-gas recirculation system, to enrich the exhaust-gas recirculation flow with NH3 in order to lower the nitrogen oxide emissions in a so-called selective non-catalytic reduction (SNCR), as per the following equation:2NH3+2NO+O2→2N2+3H2O
Owing to the poor selectivity of this reaction, it is only possible to attain NOx conversion levels of 15 to 25%, by contrast to the catalytic SCR method, in which conversion levels of over 95% are attained.
DE 103 57 402 A1 discloses a method for the operation of an internal combustion engine which has an exhaust-gas purification system with an ammonia-generating catalytic converter. Ammonia is generated, by means of the ammonia-generating catalytic converter, from the exhaust-gas flow conducted from the internal combustion engine to a turbine of an exhaust-gas turbocharger. A partial exhaust-gas flow is branched off from the ammonia-enriched exhaust-gas flow upstream of the turbine of the exhaust-gas turbocharger, said partial exhaust-gas flow being fed in on the fresh-air side of the internal combustion engine as an exhaust-gas recirculation flow. The exhaust-gas recirculation flow can be influenced by means of throttle valves that are provided, but no provision is made for a targeted supply of ammonia into the exhaust-gas recirculation flow, and there is thus also no provision made for an adaptation of the NH3 flow rate required for optimum desulphurization of the recirculated exhaust-gas flow. This has the effect that the NH3 concentration in the exhaust-gas recirculation arrangement corresponds to the NH3 concentration upstream of the SCR catalytic converter. Furthermore, in the case of fuels with high sulphur content, there is a further problem: before the exhaust-gas flow can be supplied to the fresh air, said exhaust-gas flow must be cooled because there would otherwise be the possibility of increased NOx emissions, reduced power and, in the worst case, damage to the engine. However, at the coolers required for this purpose, if temperatures of approximately 300° C. are undershot and ammonia is present, ammonium sulphate and/or ammonium hydrogen sulphate are precipitated, which lead to blockage of the cooler.
WO 2012/096123 A1 discloses an internal combustion engine which is operated with NH3 and in which an exhaust-gas recirculation flow to the fresh-air side of the compressor of the turbocharger is branched off from the exhaust-gas flow downstream of the turbine of a turbocharger. Here, an aqueous ammonia solution is injected into the exhaust-gas recirculation flow. This serves two purposes: firstly, this causes the recirculated exhaust-gas flow to be cooled, and secondly, the ammonia reacts in the combustion chamber, with the nitrogen oxides generated there, to form nitrogen in the “selective non-catalytic reduction” (SNCR) already described above. As a result of the combination of the two functions, specifically the cooling of the exhaust gas and the reduction of the nitrogen oxides by means of NH3, the partially different demands cannot be varied independently of one another, and it is certainly not possible to realize an additional, independent function such as the desulphurization through ammonium salt formation.
Both methods have in common the fact that they do not work with ammonia precursor substances such as urea, guanidinium formate, cyanuric acid or ammonium formate. The reason for this lies in the high corrosivity of the compounds and/or of the products formed therefrom, such as isocyanic acid and formic acid.
Furthermore, JP 2009-85011 A discloses a method in which, as a function of a pH value measured at the inlet side, NH3 is if required generated and supplied to the fresh-air side in order to influence the pH value.